1. Field of the Invention
The invention concerns a device for spraying liquids, especially paint and varnish, and is more particularly concerned with an improvement to the spray nozzle of a device of this kind that makes it possible to obtain under all circumstances improved shaping of the jet of sprayed liquid in a relatively broad fan shape, more particularly for a relatively low flowrate of the liquid.
2. Description of the Prior Art
There is a known liquid spray device using compressed air to spray the liquid and to give the resulting jet the required shape, that of a relatively flat fan, for example. A device of this kind is described in U.S. Pat. No. 2,646,314 and comprises a nozzle equipped with an axial liquid ejection passage, an air ejection passage that is annular, for example, and coaxial with the liquid ejection passage, oblique spray air ejection passages which converge downstream of the liquid ejection passage orifice and shaping air ejection passages the ejection axes of which lie in a plane including the axis of the liquid ejection passage. These latter passages converge towards the front of the nozzle so as to "flatten" the divergent jet of sprayed liquid and thus give it the required shape of a relatively flat fan having an approximately oblong transverse cross-section. The various air ejection passages are generally fed by two compressed air supplies, one for the shaping air ejection passages and the other for the other passages. The pressure of the first supply essentially serves to control the width of the fan while that of the second supply serves to control the fineness of the spray.
Another important parameter to be taken into account is the flowrate of the liquid. A proper choice of the number, diameter, respective positions and orientations of the various passages, in combination with the air pressures chosen, makes it possible to obtain satisfactory results for a specific range of flowrates. However, the adjustments are somewhat delicate since the actions of all the air jets interfere with each other.
It is particularly difficult to obtain a wide jet (meaning, for example, a very broad fan from 50 to 60 cm wide at a distance of 25 to 30 cm from the nozzle) with a low liquid flowrate (typically from 100 to 200 cm.sup.3 per minute). Under such circumstances, and depending on the adjustments, the problems experienced with spraying include: a jet that is too narrow, or hollow at the center, or even divided into two parts, in the vicinity of the aforementioned plane in which the axis of the liquid ejection passage is situated, or non-homogeneous spraying, specifically coarse spraying at the edge of the jet. These phenomena are attributed to the fact that at low liquid flowrates the spray air pressure has to be reduced with the result that the jet is no longer able to "resist" the action of the shaping air jets, which are necessarily violent to obtain a broad jet, despite the most appropriate choice of diameters for the various air ejection passages. A specific objective of the present invention is to overcome this problem.